Light is an important regulator of behavioral state in mammals. In addition to effects on the circadian clock, changes in lighting conditions can induce immediate changes in sleep-wakefulness. For example, light increases wakefulness in humans and induces sleep in nocturnal rodents. We have used the albino rat as a model to study acute effects of light-dark shifts on sleep patterns. Albino rats show exaggerated responses to abrupt lighting changes, particularly in their rapid eye-movement (REM) sleep patterns; following lights-off, they exhibit large increases in REM sleep (REM sleep triggering), whereas lights-on suppresses REM sleep. In addition, light acutely increases non- REM (NREM) sleep and dark increases waking in both albino and pigmented rats. During the past 4 years of funding, we have characterized acute sleep-wakefulness responses to changes in light conditions and demonstrated that lesions of the superior colliculus (SC) and pretectum (PT) attenuate these responses in albino rats. We propose an integrated series of studies to further elucidate the mechanisms by which the SC-PT region regulates sleep-wakefulness in response to lighting changes: (1) Localize areas within the SC and PT which mediate REM sleep, NREM sleep and/or waking responses to light-dark shifts; (2) Describe connections between the SC-PT and brain regions known to be involved in mediating sleep and waking; (3) Use immediate early gene expression to identify brain regions which respond to acute lighting changes and correlate with behavioral responses; and (4) Determine the effects of SCN lesions on sleep-waking responses to light-dark shifts. Results from these studies should increase our understanding of how light affects the nervous system, including effects of light therapy on sleep and mood. Furthermore, we hope to elucidate the role of the SC-PT in REM sleep regulation, which may have relevance to disorders characterized by abnormalities in REM sleep, and depression